Scientists say an epidemic of myopia, or nearsightedness, is sweeping through Asian children, and is likely due to students’ spending too much time indoors studying and not enough time outside in the sunlight.

It has long been thought that nearsightedness is mostly a hereditary problem, but researchers led by Ian Morgan of Australian National University say the data suggest that environment has a lot more to do with it.

Reporting in the journal Lancet, the authors note that up to 90% of young adults in major East Asian countries, including China, Taiwan, Japan, Singapore and South Korea, are nearsighted. The overall rate of myopia in the U.K., by contrast, is about 20% to 30%.

In Singapore, for example, rates of nearsightedness in three different ethnic groups — Chinese, Indian and Malay — have increased since 1996. Because all three groups are equally affected, says Morgan, it’s likely that some common environmental factor is driving the rise.

Studies of East Asian populations that have moved to different parts of the world are also revealing: Chinese young adults in Australia, where exposure to bright sunlight is more likely, show lower rates of myopia than Chinese young adults living in cities in East and Southeast Asia. Similarly, white children living in Sydney show lower rates of nearsightedness than those living in the U.K.

Particularly concerning is that about 10% to 20% of Asian schoolchildren suffer from high myopia, which puts them at higher risk of more serious vision problems, including blindness, in adulthood. Morgan says the culprit is the massive pressure on Asian children to succeed in school, which leads to too many hours hunched over books indoors and not nearly enough exposure to natural sunlight. Indeed, East Asian countries with high myopia rates are those that dominate international rankings of educational performance, the study notes.

Myopia, which causes people to see clearly things that are near but not those that are at a distance, is the result of elongation of the eyeball, which leads to misalignment of light on the retina. Instead of landing on the retina at the back of the eye, incoming light converges at a point in front of the retina, leading to blurry images at a distance. Animal studies show that during early development, if the eye is not allowed to regulate its size to the proper length, then myopia can occur.

The scientists think that the neurotransmitter dopamine may play a significant role in the structural development of the eyeball. Exposure to light increases the levels of dopamine in the eye, which may prevent elongation of the eyeball.

“We think there is a pretty well-confirmed mechanism,” says Morgan. “We postulate that bright outdoor light would stimulate the release of the retinal transmitter dopamine, which is known to be able to block the axial growth of the eye, which is the structural basis of myopia — the eye simply grows too big.” Animal experiments using mice and monkeys support the theory, the researchers say.

It’s not clear when the window of proper eye development closes in humans, but Morgan says it’s concerning that the high rates of nearsightedness among East Asians is occurring so early, often in elementary school. “What has happened in East Asia is that the study pressure that promotes myopia is already high for early-primary schoolkids, and they spend little time outdoors,” says Morgan. “The worst aspect of this early start is that it gives them longer to become highly myopic, because the eye continues to elongate, and then they are at risk of [more serious vision problems].”

Can the progression to myopia be prevented, or at least stopped? So far, no effective prevention methods or therapies for nearsightedness exist, other than corrective lenses like glasses or contacts. The drug atropine slows down eye growth, but the drops can cause side effects, and they lose their effectiveness over time, says Morgan. “We need more evidence on just about everything that’s been tried,” he says.

The results suggest that parents — especially tiger moms — might want to give their hard-studying children regular breaks: a couple of hours of sunlight a day would probably do it, the authors say.